The DLR and Tramlink, both the least-barriered systems in London, do have regular checks, and allegedly the DLR has long been the best-performing of all the TfL modes in terms of a low rate of fare evasion (apparently including the river services - though I still have basically no idea how anyone actually evades the fare on the boats, as they’re very clued-up on revenue).
Fare evasion tends to spike due to numerous factors, not all of which will be particularly obvious or even able to be numerically quantified. For example, Southeastern staff morale and therefore staff presence used to be pretty poor in places, but conductor and Rail Enforcement Officer mood and visibility seem to have both shot up in parallel, which has meant the company has generally done quite well over the last couple of years in matters of challenging those without tickets. The staff know what they’re doing and they’ve recently seemed quite motivated to tackle even the big problem areas. I think this will eventually reap considerable rewards for the company, and areas like their Metro routes will be less of a free-for-all. (And Tonbridge in the school run...) But I believe this isn’t actually making much of a dent in the stats as yet.
You also have the opposite, which are areas of downward decline which seem to be in a rather nasty spiral. Some of the GTR routes are now near enough 100% pay-when-challenged, with alarmingly regular reports of groups of fare evaders roaming what used to be perfectly pleasant Sussex railway lines, and it’s enough to completely wear out seasoned coppers on the rare days when they end up helping out. This isn’t just down to staff motivation though... indeed as someone posted, lines like Uckfield’s regularly get checked... it’s almost designed into the stations and environments around the stations by accident, and then also encouraged by a population which hasn’t been challenged to work out the consequences of moral values (or not) for themselves.
Between the two of these you get the routes like London Northwestern, where the company is starting to try tactics like staffing (or, rather, rostering staff for) all their ticket barriers all the time, and encouraging more revenue activity, because they do recognise that abusive fare evasion has increased and that everyone on the frontline needs a bit more encouragement and backup to even begin to sort it out. Fundamentally I don’t actually believe the south WCML is statistically that bad, but if the confidence isn’t yet there amongst the staff, people who think they can “get away with it” will start to take advantage of any weaknesses.